35<S SYSTEMATIC ZOOLOGY 





sisting of several fused vertebrae to which the pelvic girdle is 

 attached ; and this is succeeded by the caudal region. The 

 vertebrae are separated from one another by elastic discs 

 called the intervertebral ligaments. The pectoral girdle gener- 

 ally consists of only two distinct bones, the scapula, or shoulder 

 blade, and the clavicle, or collar bone, and the latter is absent 

 in many mammals ; the pelvic girdle consists of three bones, as 

 in all vertebrates above the fishes, one dorsal in position, 

 the ilium, and two ventral, an anterior, the pubis, and a pos- 

 terior, the ischium ; the posterior appendage articulates at 

 the point where these three bones meet. The bones of the 

 appendages have the same general arrangement as in all 

 the vertebrates above the fishes. 



The muscular system is well developed, and the variations in 

 the number, size, and shape of the individual muscles conform 

 to the uses to which they are put in accordance with the habits 

 of the different species. Of the nervous system we may note 

 simply that the brain attains a higher degree of development 

 here than in any other group of animals. The body-cavity of 

 the Mammalia differs from that of any other group of the Ver- 

 tebrata, in that it is completely divided into two parts by a mus- 

 cular membrane, the diaphragm, which separates it into an 

 anterior chamber, the thoracic cavity, containing the heart and 

 the lungs, and a posterior chamber, -the abdominal cavity, 

 containing the remaining viscera. 



The digestive tract consists of several regions. The mouth is 

 usually provided with teeth, which are confined to the jawbones; 

 in some cases the teeth are nearly all the same size and shape, 

 but more or less conical ; teeth of this sort constitute homodont 

 dentition, and they vary in number from one to two hundred in 

 different species. In most mammals, however, the teeth are 

 of different shapes, constituting heterodont dentition ; the most 

 anterior teeth are called incisors ; they are usually bounded by 

 the fanglike canines, one on each side in each jaw, and hack oi 

 the canines are the double teeth, of which the more anterior are 

 called premolars, the more posterior, molars. In most mammals 

 there are two sets of teeth, the milk dentition of temporary 

 teeth, which eventually fall out, and the permanent teeth, which 

 succeed them ; there are rarely more than forty-four of the 



